The present invention relates to semiconductor MOS devices and more specifically to a high voltage metal oxide device with an enhanced n-well region.
When designing high voltage metal oxide (MOS) devices two criteria must be kept in mind. First, the device should have a very high breakdown voltage (VBD). Second, the device, when operating, should have as low an on-resistance (RDSON) as possible. One problem is that techniques and structures that tend to maximize VBD tend to adversely affect RDSON and vice versa.
To overcome this problem, different designs have been proposed to form devices with acceptable combinations of VBD and RDSON. One such family of devices is fabricated according to the reduced surface field (RESURF) principal. These devices utilize an extended drain region (in one embodiment a n-well) to support high off-state voltage (VBD). These devices have a maximum number of charges in the drain area of about 1xc3x971012cmxe2x88x922 before avalanche breakdown occurs. This maximum charge sets up the lowest RDSON possible since RDSON is proportional to the charge in the drain region.
To help alleviate this problem, some devices utilize a top layer of a conductivity type opposite the extended drain region (in one embodiment a p-type layer) inside the drain region. The top layer allows for a drain region having approximately double the charge than previous designs, which decreases the RDSON. The top layer helps to deplete the extended drain when the extended drain is supporting high voltage, thus allowing for high breakdown voltage.
One drawback to this approach is that a high drain concentration under the gate region and adjacent to the channel region can lead to premature breakdown when the device is blocking voltage. Thus, what is needed is a drain region that has a high concentration in most areas but provides for lower concentration under a gate region.